1. Field of the Invention
The present invention relates to an inorganic composition containing particles of a minute particle diameter and having a good mechanical strength and an excellent surface roughness which is little altered after a surface treatment with acid or alkali, and furthermore having a thermal expansion property well suitable with other materials. Especially, this invention relates to a substrate for a magnetic recording medium used in various magnetic information recording devices, in particular, to a disc-shaped substrate for information recording medium having a surface ultra-smoothness, a washing property and a high strength in a perpendicular magnetic recording medium. In the present invention, the term “information recording medium” denotes a magnetic information recording medium usable in a built-in hard disc, a removal hard disc, and a card-type hard disc used as a hard disc of personal computer; a hard disc for a digital video camera and a digital camera and an audio equipment; a hard disc for automobile navigation, and a hard disc for mobile phone.
2. Background Art
Recently, a large volume of data such as animation and sound have become handled with a multimedia personal computer, a digital video camera and a digital camera so that a magnetic information recording apparatus with a high-capacity has become necessary. As a result, in order to increase the areal density of a magnetic information recording medium, there is a tendency to increase the bit and track densities thereof while reducing the bit cell size. For corresponding with this tendency, along with the reduction of the bit cell size, a magnetic head becomes to operate in a state closer to a disc surface. Thus, when a magnetic head operates in a low floating state to a magnetic information recording medium disc substrate or in a state in contact therewith, it becomes important to develop, as a technique for starting and stopping a magnetic head, a technique such as a landing zone method in which an adhesion preventive treatment (a texture processing and so on) is performed at a specific portion of a magnetic information recording medium disc substrate (a non-recording portion on the inside or outside diameter side of a disc) to perform the starting and stopping of the magnetic head at that portion.
In the currently used magnetic information recording device, the CSS (contact-start-stop) method is used in which the operation is repeated such that a magnetic head is in contact with a magnetic information recording medium disc substrate prior to starting the device, and floats from the substrate when the device is started. In this case, when the both contact-making surfaces are more than necessary smooth like a mirror, the stiction occurs between them so as to cause problems such as a non-smooth rotation starting and a damage of the magnetic information recording medium surface along with the increase in the friction index. Thus, for the magnetic information recording medium disc substrate, the contradicting requirements are demanded, that is, a low-floating of the magnetic head along with the increase in the recording capacity and a prevention of stiction of the magnetic head on the magnetic information recording medium disc substrate. For these contradicting requirements, a landing zone technique has been developed to provide a starting/stopping portion for the magnetic head in a specific region on the magnetic information recording medium disc substrate.
Furthermore, when the recording density exceeds 100 Gb/in2, the substrate becomes thermo-unstable (heat fluctuation) with such a minute magnetization unit, so that the longitudinal recording system reaches a physical limit for the demand for a high density recording exceeding 100 Gb/in2.
For solving these problems, a perpendicular magnetic recording system has been adopted, which makes the easy magnetization axis perpendicular so as to be capable of reducing the bit size in the extreme and also expecting the reduction of the anti-magnetic field as well as the effect by the shape magnetic anisotropy owing to having a predetermined medium film thickness (from 5 to 10-fold of the longitudinal recording system), enabling the solution of problems such as the decrease in the recording energy and heat fluctuation of the substrate occurring in the high density of the conventional longitudinal magnetic recording system and the realization of superior improvement of the recording density compared to the conventional longitudinal magnetic recording system. As a result, in the perpendicular magnetic recording system, it has already become possible to practically achieve the recording density of 100 Gb/in2 or more at the mass production level, and researches on the recording density exceeding 300 Gb/in2 have been already under way.
In this perpendicular magnetic recording system, magnetization is performed perpendicularly to the medium surface, such that a medium having the easy magnetization axis in the perpendicular direction is used different from a conventional medium having the easy magnetization axis in the longitudinal direction. As a recording layer of the perpendicular magnetic recording system, chromium alloys, such as CoCrPt, CoCrPt—Si, and CoCrPt—SiO2; and iron alloy such as FePt have been researched and put to practical use.
However, for such a medium of a metallic oxide type including FePt and the like, it is necessary to raise the film formation temperature for the miniaturization of the magnetic body crystalline particles and perpendicular formation thereof. Furthermore, in a certain recent study, annealing is performed some times at high temperatures (in the range from 300 to 900° C.) to improve the magnetic property. Therefore, a substrate material must be sustainable such a high temperature, and must not generate a deformation of the substrate and a deterioration of the surface roughness thereof.
In addition, along with the magnetic recording density improvement, the perpendicular magnetic recording medium also tends towards a low-floating type with the head-floating height of 15 nm or less, and, furthermore, a near-contact recording type or a contact-recording type. On the other hand, in order to efficiently use the medium surface as a data region, the medium system has become a ramp-loading system from the system provided with the conventional landing region. Accordingly, the data region of a disc surface or the entire surface of a substrate surface must be ultra-smooth and flat so as to enable the reduction in this floating height and the contact recording.
Furthermore, these magnetic recording medium substrates must have none of crystal anisotropy, foreign substance, impurity, and such to affect the film-forming medium crystals, have precise, homogeneous and minute structure, and a chemical tolerance against washing and etching with various drugs.
And, these days, for speeding up information processing, technical development has been in progress by rotating the magnetic information recording medium disc substrate of a magnetic recording device at high speed. However, rotation at high speed may cause a deflection and deformation of the substrate, such that a high mechanical strength is required for the substrate material. In addition, for the current built-in magnetic information recording device, a magnetic information recording device with a removable disc system and a card disc system is on the stage of investigation and practical use, the application development to digital video camera, digital camera, and so one being started.
Although, traditionally, as a magnetic disc substrate material, aluminum alloy has been used a great correspond, substrate made of aluminum alloy is apt to produce projections or spot-like unevenness on the substrate surface in the polishing process, so that a satisfactory substrate in terms of flatness and smoothness is hardly obtainable. And, aluminum alloy is a soft material and easily deformed so as to hardly correspond with a tendency of ultra-slim substrate. Furthermore, aluminum alloy has problems such as causing head crash due to deflection during a high-speed rotation resulting in the medium damage so as not to be a material sufficiently corresponding with the high density recording in the future. Furthermore, since its allowable temperature limit during the film formation, the most important feature in a magnetic recording system, is 300° C. or less, the substrate becomes thermally deformed when the film formation is performed at 300° C. or more, or when annealing is conducted at high temperatures such as from 500 to 900° C. Therefore, aluminum alloy is hardly applicable to a substrate used in the magnetic recording medium requiring such a high temperature processing.
As a material to solve problems involved in the aluminum alloy substrate, the chemically reinforced soda lime glass (SiO2—CaO—Na2O) and aluminosilicate glass (SiO2—Al2O3—Na2O) have been known.
In these cases, since polishing is performed after the chemical reinforcement treatment, the number of insecure elements in the reinforced layer is high in the slim disc formation, and also the heat resistance of the substrate itself is low. That is, after the film formation on a given sample by heating the magnetic recording medium at 300° C. or more, flatness measured by a predetermined method becomes ruined. As a result, there cause problems such as a deformation after the medium film formation, an elution of alkaline component from the inside of the substrate to cause the film damage, and a deterioration of the reinforced layer and non-reinforced layer.
And as a material to overcome shortcomings of the above-described chemically reinforced glass substrate, there have been developed various glass ceramics, such as a crystallized glass of an SiO2—Li2O—P2O5 type containing lithium disilicate crystals (Li2Si2O5) and α-quartz (α-SiO2) crystals; and a crystallized glass of an SiO2—Al2O3—Li2O type containing lithium disilicate crystals (Li2Si2O5) and β-spodumene (LiAlSi2O6) crystals (for example, Patent documents 1 to 11, etc.).
Patent Document 1: Japanese Unexamined Patent Application, Publication No. Sho 62-72547,
Patent Document 2: Japanese Unexamined Patent Application, Publication No. Hei 06-329440,
Patent Document 3: Japanese Unexamined Patent Application, Publication No. Hei 07-169048,
Patent Document 4: Japanese Unexamined Patent Application, Publication No. Hei 09-35234,
Patent Document 5: U.S. Pat. No. 5,336,643,
Patent Document 6: U.S. Pat. No. 5,028,567,
Patent Document 7: Japanese Unexamined Patent Application, Publication No. Hei 10-45426,
Patent Document 8: Japanese Unexamined Patent Application, Publication No. Hei 11-16143,
Patent Document 9: Japanese Unexamined Patent Application, Publication No. 2000-233941,
Patent Document 10: Japanese Unexamined Patent Application, Publication No. 2000-302481, and
Patent Document 11: Japanese Unexamined Patent Application, Publication No. 2001-184624.
However, a crystallized glass of a Li2O—Al2O3—SiO2 type according to Patent Document 1 has lithium disilicate (Li2Si2O5) and α-cristobalite as a crystalline phase, advantageously controls the thermal expansion coefficient thereof by separating the above-described crystalline phase, and is able to obtain a magnetic disc substrate with a high strength. However, it cannot sufficiently correspond with a tendency of low-floating head along with the recording capacity improvement in a great and rapid progress for the targeted surface roughness (Ra).
A crystallized glass of an SiO2—Li2O—MgO—P2O5 type according to Patent Document 2 has lithium disilicate (Li2Si2O5) and α-quartz (α-SiO2) as a crystalline phase, and does not need the conventional mechanical texture and chemical texture by controlling the spherical particle size of α-quartz (α-SiO2) thereby making it possible to control the surface roughness (Ra) after polishing in the range from 15 to 50 Å so as to be a very excellent material as an entire surface texture material of the substrate surface. However, the targeted surface roughness (Ra) thereof is too large compared to 10 Å or less such that it cannot sufficiently correspond with a tendency of low-floating head along with the recording capacity improvement in a great and rapid progress. Furthermore, it has a low thermal resistance similarly as the reinforced glass so as to pose problems such as the substrate deformation after the medium film formation and annealing as well as alteration of the surface roughness.
Although in Patent Document 3 is disclosed a photosensitive crystallized glass of an SiO2—Li2O type containing photosensitive metals such as Au and Ag, and in Patent Document 4 is disclosed a substrate for use in the magnetic disc comprising lithium disilicate (Li2Si2O5) and β-spodumene (LiAlSi2O6) in a glass of a SiO2—Al2O3—Li2O type respectively, both of these glass ceramics substrate materials have a low heat resistance, posing problems, such as deformation of the substrate after the film formation and annealing as well as alteration of the surface roughness.
And, in Patent Document 4 is described a substrate for use in the magnetic disc with a principal crystalline phase comprising lithium disilicate (Li2Si2O5) and β-spodumene (LiAlSi2O6) in a glass of a SiO2—Al2O3—Li2O type, in which β-eucryptite (Li2Al2Si2O8) has been separated by newly performing the above-described crystallization heat treatment at lower temperatures (from 680 to 770° C.). However, although these crystallized glass substrates elute less alkali compared with chemically reinforced amorphous glass, the alkali elution still occurs, and even these crystallized glass substrates are posing problems such as a “decrease in the magnetic property of the recording medium,” an “adhesion of defect to the substrate surface,” and a “record missing” caused by the alkali elution.
And, although in Patent Document 5 is disclosed a low expansion transparent crystallized glass of a SiO2—Al2O3—Li2O type, and in Patent Document 6 is disclosed a crystallized glass of a SiO2—Al2O3—ZnO type, respectively, both of these glass ceramics materials were not examined and suggested as a substrate material for use in a perpendicular magnetic recording medium for the heat resistance (that is, the degree of flatness measured by a predetermined method after the exposure to a specified temperature environment (at 500° C. or more for 5 minutes or more) at all. The particularly important maintenance of ultra-flatness of the substrate surface after the film formation and annealing at high temperature was not discussed at all.
In Patent Documents 7 and 8 is disclosed a technique of preparing a crystallized glass for a laser texture by separating, from a crystallized glass of an SiO2—Li2O—K2O—P2O5—Al2O3 type, lithium disilicate (Li2Si2O5), mixed crystals of lithium disilicate and α-quartz (α-SiO2), or mixed crystals of lithium disilicate and α-cristobalite (α-SiO2), or mixed crystals of lithium disilicate, α-cristobalite and α-quartz (α-SiO2) as a crystalline phase. And in Patent Document 10 is disclosed a similar technique of preparing a crystallized glass for a laser texture by separating, from a crystallized glass of an SiO2—Li2O—P2O5—Al2O3 type, the above-described lithium disilicate and those mixed crystals. However, the targeted surface roughness Ra (an arithmetic mean roughness) these days is 10 Å or less, preferably 5.0 Å or less, more preferably 3.0 Å or less, and most preferably 2.0 Å or less, so that these glass ceramics cannot sufficiently correspond with the tendency of a low floating head along with the recording capacity improvement making rapid progress.
In Patent Document 9 is disclosed a technique of preparing a crystallized glass substrate for high recording density by separating, in a crystallized glass of an SiO2—Li2O—K2O—P2O5—ZrO2—Al2O3 type, lithium disilicate (Li2Si2O5), mixed crystals of lithium disilicate and α-quartz (α-SiO2), mixed crystals of lithium disilicate and α-cristobalite (α-SiO2), or mixed crystals of lithium disilicate, α-cristobalite, and α-quartz; while in Patent Document 11 is disclosed a similar technique of preparing a crystallized glass substrate for high recording density (characterized) by separating, in a crystallized glass of an SiO2—Li2O—K2O—P2O5—ZrO2—Al2O3—Na2O type, mixed crystals of lithium disilicate and α-quartz (α-SiO2), and mixed crystals of lithium disilicate and β-spodumene (Li2Al2Si4O12). Although the crystallized glass manufactured by this technique can secure a flatness and smoothness on atomic level by polishing processing, it poses problems in surface property such as a significant alteration of smoothness and a formation of minute projections as the effect of various washings performed in the process of magnetic film formation.
Since these glass ceramics are different in hardness between the crystalline phase and amorphous phase, minute concaves and convexes are irreversibly generated between these two phases after the polishing processing, thereby these concaves and convexes having been playing a role in preventing a magnetic head from adhering to a disc substrate.
On the other hand, in a magnetic information recording device, in response to the aforementioned landing zone technique, a ramp load technique (contact recording with a magnetic head) has been developed in which a portion for start/stop of the head is removed from the magnetic information recording medium disc substrate so that concaves and convexes preventing the magnetic head from adhering to the disc substrate has become unnecessary. Therefore, it has become possible that the magnetic head operates in a state extremely close to the information recording medium surface by making the substrate surface ultra-flat and smooth, and that the bit cell size is reduced and the recording density is increased.
Accordingly, in order to prevent a head and medium from damage even at such an extremely low floating height or in a state in which a head is in contact with a medium, the surface roughness Ra (an arithmetic mean roughness) of a substrate is preferably made to be not more than 10 Å. However, to obtain such an ultra-fine and smooth polished surface, a substrate comprising crystals having minute mean crystalline particle diameter is demanded.
Also, along with an increase in the recording density, a high precision is required in positioning a magnetic head and a medium, so that a high dimensional accuracy is required for each component of the disc substrate and magnetic information recording device. Consequently, since the effect of differences in the mean thermal expansion coefficients on these components becomes not negligible, differences in these mean thermal expansion coefficients must be made as little as possible. More strictly, it is often preferable that a mean coefficient of thermal expansion of a disc substrate is very slightly larger than a mean coefficient of linear expansion of these components.
As a component used in a small-sized magnetic information recording medium in particular, the one having the thermal expansion coefficient from +90 to +100 (×10−7·° C.−1) is often used, and it is thought it necessary that the disc substrate also has a thermal expansion coefficient in this range, causing trouble of the generation of writing error even with a confusion of thermal expansion coefficient as small as 1 (×10−7·° C.−1). However, the range of thermal expansion coefficient has become wider by a design in the drive so as to have a certain degree of freedom therein so that a component with a low thermal expansion coefficient has become usable in the drive design. That is, even a composition having the thermal expansion coefficient in the range from +60 to +80 (×10−7·° C.−1) has become usable in the component.
An objective of the present invention is to provide an inorganic composition used in an information recording medium and such which has an excellent surface property capable of sufficiently corresponding with a ramp load system for a high density recording in both of the longitudinal magnetic recording system and perpendicular magnetic recording system in response to the above-described design improvement of magnetic information recording devices; a high strength durable to high speed rotation; both of the heat expansion property and heat resistance suitable with each drive element; a low melting point; and a high productivity.